Due to the importance of hydrogenation of graphene for several applications, we present an alternative approach to hydrogenate graphene based on density functional theory calculations. We find that a negative perpendicular electric field F can act as a catalyst to reduce the energy barrier for molecular H2 dissociative adsorption on graphene. Increasing -F above 0.02 a.u. (1 a.u.=5.14×1011 V/m), this hydrogenation process occurs smoothly without any potential barrier.